This invention relates generally to a method and apparatus for making pulsed nuclear magnetic resonance (NMR) measurements of earth formations while drilling a borehole. More specifically, the invention is directed to a NMR measurement-while-drilling (MWD) tool having the required mechanical strength and measurement sensitivity, and a method and apparatus for monitoring the motion of the measuring tool in order to take this motion into account when processing NMR signals from the formation surrounding the borehole.
Various methods exist for performing downhole measurements of petrophysical parameters in geologic formations. Pulsed NMR logging is among the most important of these methods, and was developed primarily for determining parameters such as formation porosity, fluid composition, the quantity of movable fluid, permeability, and others. Importantly, NMR measurements are environmentally safe and are unaffected by variations in the matrix mineralogy.
In a typical NMR measurement, a logging tool (measurement device) is lowered into a drilled borehole to measure properties of the geologic formation near the tool. Then, the tool is extracted at a known rate while continuously taking and recording measurements. At the end of the experiment, a log is generated showing the properties of the geologic formation along the length of the borehole. This invention relates primarily to an alternative measurement, in which pulsed NMR logging can be done while the borehole is being drilled. The advantages of the latter approach in terms of saving both time and costs are apparent. Yet, very little has been done so far in terms of developing practical NMR logging-while-drilling (LWD) or measurements-while-drilling (MWD) solutions. Two of the main stumbling blocks appear to be the very stringent requirements concerning the mechanical strength of the device, as well as problems associated with inaccuracies of the received signals due to motions of the tool. The present invention addresses successfully both issues and therefore is believed to make a significant contribution over the prior art.
In order to more fully appreciate the issues discussed in detail next, a brief overview of NMR methods for measuring characteristics of formations surrounding a borehole is presented first. The interested reader is directed, for example, to the following article: Bill Kenyon et al., Nuclear Magnetic Resonance Imagingxe2x80x94Technology for the 21st Century, OILFIELD REV., Autumn 1995, at 19, for a more comprehensive review. The Kenyon article is incorporated herein by reference.
Basically, in the field of NMR measurements of earth formations surrounding a borehole, a downhole static magnetic field is used to align the magnetic moment of spinning hydrogen (H) protons in the formation in a first direction, the direction of the static magnetic field. In order to establish thermal equilibrium, the hydrogen protons must be exposed to the polarizing field for a multiple of the characteristic relaxation time, T1. Then, the magnetic component of a radio frequency (RF) electromagnetic wave pulse, which is polarized in a second direction orthogonal to the static field, is used to tip the protons to align them in a third direction that is orthogonal to both the first and the second direction. This initial RF pulse is thus called a 90xc2x0 pulse. Following the 90xc2x0 pulse the protons in the formation begin to precess about the axis of the first direction. As a result, the protons produce an oscillating magnetic field. If an antenna is placed into the oscillating magnetic field, the oscillating magnetic field will produce an oscillating electric current in the antenna. Because the amplitude of the induced electrical signal is proportional to the porosity of the earth formation being measured, the signal may be calibrated to measure formation porosity. However, due to dephasing and irreversible molecular processes, the induced signal decays rapidly after the RF pulse is removed. Consequently, when the antenna is used both to transmit the RF pulses and to receive the induced NMR signal as in one embodiment of this invention, this first NMR signal may not be observable because the antenna electronics are still saturated from residual effects of the 90xc2x0 RF pulse. Therefore, the NMR signal must be rebuilt as a spin echo, as discussed below, so that it may be measured.
Additionally, the behavioral characteristics of the protons after removal of the RF pulse can be used to garner information about other formation properties, such as pore size distribution and permeability. Immediately after the 90xc2x0 RF pulse is turned off, the protons precess in phase. However, due to inhomogeneities in the static magnetic field and irreversible molecular processes, the protons begin to dephase, which causes the induced signal to decay. Nevertheless, the dephasing due to inhomogeneities in the static magnetic field is partially reversible. Therefore, by applying a 180xc2x0 RF pulse, the instantaneous phases are reversed such that the protons gradually come back into phase, thus rebuilding the induced signal. The antenna can detect this signal because the time required for rebuilding the signal is long enough to allow the antenna electronics to recover from the 180xc2x0 RF pulse. After the signal peaks at the time when the protons are back in phase, the signal will then begin to decay due to dephasing in the opposite direction. Thus, another 180xc2x0 RF pulse is needed to again reverse the instantaneous phases and thereby rebuild the signal. By repeating a series of 180xc2x0 RF pulses, the signal is periodically rebuilt after each dephasing, although each rebuilding is to a slightly lesser peak amplitude due to the irreversible molecular processes. Eventually, the irreversible processes prevail such that no further rephasing is possible and the signal dies out completely. Each rebuilding of the signal in this manner is called a spin echo, and the time constant associated with the decay of the spin echo amplitudes is called the transverse relaxation time, T2.
Because experiments have shown that T2 is proportional to the pore size of the formation, calibration and decomposition of T2 yields a measure of the formation""s pore size distribution. Moreover, when combined with the porosity measurement, T2 yields an estimate of the formation""s permeability. As noted above, the NMR signal may also be calibrated to obtain other formation characteristics, such as free fluid volume, bound fluid volume, fluid identification, and diffusion coefficients. Because the drilling mode of operation using a preferred embodiment of this invention may allow for enough time to develop only one spin echo, or at most a few spin echos, the apparatus may achieve only porosity and limited T2 measurements while drilling. However, the other types of NMR measurements discussed above are possible in non-drilling modes of operation, such as stationary tool, sliding or wiping tool.
From the preceding discussion it should be apparent that in order to enable accurate NMR measurements it is important that the same protons be tipped and rephased for each successive spin echo. Excessive movement of the tool in the borehole during NMR measurement can destroy the accuracy of the measurement by changing the location of the measurement volume, i.e., which protons in the formation are affected by the interaction of the static and RF pulse magnetic fields. Therefore, if the motion of the tool during NMR measurement is not known, which generally is the case in a logging-while-drilling environment, the NMR measurement may not be reliable.
The present inventors know of three issued patents directed to practical NMR measurements while drilling: U.S. Pat. No. 5,705,927 issued Jan. 6, 1998, to Sezginer et al.; U.S. Pat. No. 5,557,201 issued Sep. 17, 1996, to Kleinberg et al.; and U.S. Pat. No. 5,280,243 issued Jan. 18, 1994, to Miller. Of these references, the ""201 patent more specifically shows how to improve the tool""s susceptibility to lateral tool motion by increasing the radial dimension of the measurement volume. However, the axial length of the sensitive zone (i.e., the measurement volume) of the ""201 patent is on the order of two to four inches, whereas that of the present invention is on the order of two feet. Thus, the susceptibility of the present invention to axial tool movement is greatly improved over the prior art. Indeed, for typical drilling rates, axial movement of the present NMR tool has a negligible impact on the quality of the NMR measurement. Importantly, none of the three patents recited above discloses any means to monitor the tool motion to assure a drilling operator that the NMR measurements are accurate.
One method of dealing with the motion of the NMR tool in accordance with the present invention is to monitor the tool motion during NMR measurement and discard the measurement if the tool motion is above maximum acceptable limits. For example, in a preferred embodiment of this invention, testing has shown that the lateral velocity of the tool must be less than or equal to about 0.2 m/s to preserve the integrity of the NMR measurement. Accordingly, an important aspect of the present invention is the disclosure of a method for monitoring the tool motion by using two pairs of accelerometers located at the ends of two coplanar, orthogonal drill collar diameters. The accelerometers are used to measure the lateral acceleration of the tool, and the acceleration is integrated once to obtain the velocity and twice to obtain the displacement.
Although U.S. Pat. No. 4,958,125, issued Sep. 18, 1990, to Jardine et al., discloses a similar method and apparatus for determining characteristics of the movement of a rotating drill string, the method for determining lateral acceleration in the ""125 patent is directed to a vertical drill string orientation. Referring to FIG. 8, four accelerometers oriented such that ac1 and ac2 are on one axis and ac3 and ac4 are on an orthogonal axis, both orthogonal to the general axis of rotation, the ""125 patent sets forth the following equations of motion:
ac1=ac+ax cos d
ac2=acxe2x88x92ax cos d
ac3=ac+ax sin d
ac4=acxe2x88x92ax sin dxe2x80x83xe2x80x83Eqs. [1]
where ac is the centripetal acceleration, ax is the lateral acceleration, and d is the angle between the ac1/ac2 axis and the lateral acceleration vector. From Eqs. [1], the ""125 patent derives the following expressions for the rotation speed, S, and lateral acceleration, ax:
S=[60/2xcfx80]*[(ac1+ac2)/(2r)]xc2xdxe2x80x83xe2x80x83Eq. [2]
ax{[(ac1xe2x88x92ac2)/(2)]2+[(ac3xe2x88x92ac4)/(2)]2}xc2xdxe2x80x83xe2x80x83Eq. [3]
The direction of the lateral acceleration is determined by the following expression:
tan d=(ac3xe2x88x92ac4)/(ac1xe2x88x92ac2)xe2x80x83xe2x80x83Eq. [4]
However, Eqs. [1] do not contain any gravitational acceleration terms. Thus, Eqs. [1] correctly describe the tool motion only if the tool is oriented vertically such that the lateral component of the gravitational acceleration is zero.
To describe the tool motion accurately if the tool is in some general, inclined orientation, the equations of motion must include the gravitational acceleration terms as follows:
ac1=ac+ax cos d+g sin xcex1 cos e
ac2=acxe2x88x92ax cos dxe2x88x92g sin xcex1 cos e
ac3=ac+ax sin dxe2x88x92g sin xcex1 sin e
ac4=acxe2x88x92ax sin d+g sin xcex1 sin exe2x80x83xe2x80x83Eqs. [5]
where g is the earth""s gravitational constant (9.81 m/s2), xcex1 is the inclination angle of the tool axis with respect to the vertical (as shown in FIG. 7), and e is the angle between the a1/a2 axis and the g sin xcex1 direction (as shown in FIG. 8). As noted in the ""125 patent, Eq. [2] still holds true for a general orientation because of fortuitous plus and minus signs on the gravitational acceleration terms. However, neither Eq. [3] nor Eq. [4] holds true for a general orientation. Thus, for a general orientation, another method is needed to determine the magnitude and direction of the lateral acceleration, ax.
In a specific embodiment, the present invention solves this complication caused by the presence of the gravitational acceleration terms in a general, inclined drill string orientation by incorporating a high-pass filter to eliminate those terms. This solution is possible because the rotational frequencies of typical drilling speeds are well below the frequencies of the lateral accelerations of interest. Thus, the gravitational acceleration terms, which vary periodically at the frequency of the drill string rotation speed, can be safely eliminated without corrupting the lateral acceleration signals. After filtering in this manner, the governing equations of motion revert back to Eqs. [1], and Eqs. [3] and [4] may be used to determine the magnitude and direction of the lateral acceleration. Then, the lateral velocity and lateral displacement may be obtained by integrating the lateral acceleration once and twice, respectively. By comparing the measured motion to the allowable motion criterion, it is possible to modify the NMR measurement to optimally suit a given drilling environment.
Based on information from the motion sensors and based on parameters set by the operator before the tool was deployed, in accordance with the present invention the tool enters one of the following operating modes:
(a) Shutdown. This mode is selected anytime the tool detects the presence of metallic casing and/or is on the surface, or detects motion phenomena that make NMR measurements impossible.
(b) Wireline emulation. When no motion is detected, the tool attempts to emulate NMR measurements as typically done by wireline NMR tools.
(c) Normal drilling. During normal drilling conditions, moderate lateral motion is present, which allows for abbreviated NMR measurements.
(d) Whirling. During whirling, lateral motion is violent, but short time windows exist during which the lateral velocity drops to a point, where a porosity-only measurement is possible. The tool identifies these windows and synchronizes the NMR measurement appropriately.
(e) Stick-slip. In this drilling mode, windows exist in which short NMR measurements are possible, interspersed with periods of very high lateral/rotational motion. Again, the tool identifies these windows and synchronizes the NMR measurement appropriately.
The motion management aspect of this invention provides an algorithm to predict desirable time windows in which to make valid NMR measurements. As seen in FIG. 9, experiments have identified three distinct types of tool motion: (1) normal drilling; (2) whirling; and (3) stick-slip. These three types of motion are identifiable based on the time histories of the rotation speed, lateral velocity, and lateral displacement of the tool. In normal drilling motion, the lateral velocity of the tool is typically within acceptable limits so that valid NMR measurements may be made at almost any time. Sample plots of typical tool motion during normal drilling are shown in FIGS. 10A(1)-(6) and 10B(1)-(6). In whirling motion, the lateral velocities are usually outside acceptable limits, which makes valid NMR measurements difficult to obtain. However, as shown in FIG. 9, by monitoring the velocity, acceleration, and time duration in which the velocity remains within certain prescribed limits, it is possible to predict acceptable NMR measurement periods during whirling. Sample plots of typical tool motion during whirling are shown in FIGS. 13A(1)-(6) and 13B(1)-(6). In stick-slip motion, the drill bit periodically sticks to the borehole wall and then slips away, causing the drill string to periodically torque up and then spin free. Sample plots of typical tool motion during stick-slip are shown in FIGS. 11A(1)-(6); 11B(1)-(6); 12A(1)xe2x88x9d(6) and 12B(1)-(6). During the stick phase, the tool is virtually stationary thus providing a good time window in which to make NMR measurements. In contrast, the lateral velocity during the slip phase may be outside acceptable limits, depending on such variables as bit type, formation strength, and stiffness and length of the drill string. By analyzing the time histories in this manner, acceptable time windows may be predicted in which to make valid NMR measurements.
The motion identification aspect of this invention also provides another benefit with regard to drilling tool damage reduction and service life enhancement. Because whirling and stick-slip motion can be damaging to drilling tools, this invention""s capability of identifying these types of motion is very useful to a drilling operator. Specifically, if the operator knows that the drill collar is undergoing whirling or stick-slip motion, the operator can make appropriate changes to the weight-on-bit and rotation speed parameters and thereby change the motion to approach normal drilling as much as possible. As a result, tool damage is reduced and service life is enhanced.
Alternatively, this invention also incorporates a method for measurement of the tool motion by means of acoustic sensors. In this alternative, at least two acoustic sensors are placed on the perimeter of the drill collar. If only two acoustic sensors are used, they are preferably placed on orthogonal diameters (i.e., spaced 90xc2x0 apart). These acoustic sensors detect the distance from the tool to the borehole wall and thus directly measure the lateral displacement of the tool in the borehole. Because the sampling rate of the acoustic sensors is much greater than typical drill string rotation speeds, the rotation is negligible with respect to the displacement calculation. In turn, the displacement may be differentiated once to obtain the velocity and twice to obtain the acceleration. Again, by comparing the measured tool motion to the allowable motion criterion, the corresponding NMR measurement may be appropriately retained or discarded. This quality control check may be accomplished using either displacement or velocity data, because the displacement criterion is over a known time span (i.e., the time between the 90xc2x0 pulse and the signal acquisition window). Alternatively, when combined with an inclinometer or magnetometer measurement from which the drill collar rotation speed may be obtained (as described in U.S. Pat. No. 4,665,511 issued May 12, 1987, to Rodney and Birchak), the time histories of the motion may be used in conjunction with the prediction algorithm discussed above to predict acceptable NMR measurement windows. The same approach could also be used with contact sensors (as described in U.S. Pat. No. 5,501,285 issued Mar. 26, 1996, to Lamine and Langeveld) instead of acoustic sensors to measure the tool displacement by measuring the change in resistance through the drilling mud.
Another alternative for the motion management aspect of this invention is to correct the NMR measurement for losses in the NMR signal due to lateral motion during measurement. That is, by measuring the lateral displacement of the tool during NMR measurement as described above, the change in the sensitive volume can be calculated, which in turn allows the calculation of an appropriate correction factor to be applied to the received NMR signal to compensate for tool movement. This correction may be accomplished by using displacement information derived from any of the three sensor types discussed above (accelerometers, acoustic sensors, or contact sensors). An advantageous embodiment of the acoustic sensor form of this invention meets a tool position error specification of about 5%, which allows correction of the NMR signal to within about 95% of the signal for a stationary tool. Therefore, rather than discard an NMR measurement taken during a period of what would otherwise be excessive lateral motion, the NMR signal can be corrected to compensate for the tool motion.
Another aspect of the motion-detection problem of this invention is the operation of phase-alternated signal averaging, which is typical for NMR data acquisition and which is further described below. The salient feature of phase alternation is the coherent accumulation of NMR data, coupled with the progressive suppression of non-NMR artifacts. A large contribution to the latter comes from magneto-acoustic oscillations (xe2x80x9cringingxe2x80x9d) within the ferrous material as well as pulse-induced vibrations in current-carrying conductors (also customarily termed xe2x80x9cringingxe2x80x9d). The pair-wise subtraction process relies on the fact that these artifacts are more or less repeatable, given the same excitation through a series of 180xc2x0 RF pulses. It has been determined by experimentation that the patterns of these artifacts tend to change cyclically with the tool""s orientation and bending.
This invention provides for a means to accommodate ringing pattern changes and the effects of partial de-coupling of the magnet and antenna by taking advantage of the fact that the generally cyclical and repetitive nature of the drilling process is the source of the dynamic geometry changes. Practically all drilling involves rotation of a drill bit. Rotation is provided by bulk rotation of the entire drill string, use of a mud motor, or by a combination of both. As an example, the bit center orbit plots 10B3, 11B3, 12B3 and 13B3 show that the position of the bit is very repetitive and essentially duplicated with each revolution for drilling modes other than whirling. By measuring one of the many manifestations of this rotation, including oscillations of the on-rotating section, the NMR measurement can be repeatedly synchronized to a particular and repeatable geometry.
Magnetic and gravity tool face angles are among the most available means for tracking tool orientation on a rotating or oscillating drill string. Bending stress, position derived from integration of acceleration data, sonic sensors or contact sensors are other examples of sensors suitable for synchronization of the NMR measurement.
In an important aspect, this invention also provides a permanent magnet, preferably having tubular construction, which produces a static magnetic field that is oriented in a substantially orthogonal direction of both the axis of the borehole and the drilling device, and that diminishes in magnitude by about the square of the distance from the magnet. Over the relatively thin sensitive volume (1.5 mm thickness) the radial field gradient is essentially constant.
As shown in FIG. 6B, the magnetic field is that of a linear dipole with a field direction that depends on the azimuthal direction of the tool. Although some prior NMR tools have used magnets having a linear dipole magnetic field (such as that disclosed in U.S. Pat. No. 4,710,713), those tools did not comprise a tubular magnet through which drilling mud may be pumped nor are these magnets meant to be rotated. Additionally, although the Kleinberg et al. ""201 patent discussed above comprises tubular magnets, the magnetic field produced is not a linear dipole. The tubular construction used in accordance with the present invention provides a central cavity through which drilling mud may be pumped to enable NMR measurement while drilling. Also, the magnetic field produced by the magnet has an essentially constant gradient within the measurement volume which, when combined with an RF pulse that is tuned to the proper frequency, produces a more uniform annular sensitive region (measurement volume) that is completely within the earth formation.
Another aspect of the present invention is the use of a magnet that is comprised of multiple segments. Thus, in accordance with the present invention it is possible to tailor the resultant field by tuning the contributions from the individual segments. Such tuning may be accomplished by (a) selective demagnetization, which is possible for the SmCo5 variant of samarium-cobalt material, by (b) adjusting the volume for each segment, or (c) by adjusting the polarization direction of each individual segment. Such freedom in field shaping is advantageous if it is necessary, as in the present invention, to pre-compensate the magnetic field in order to accommodate the effects of soft-magnetic material in the vicinity of the magnet.
It may not be obvious even to a person skilled in the art that the magnetic field from a linear dipole is suitable for measurements while drilling, where the magnet is rotated with respect to the formation at rates up to about 300 RPM. Any given point within the sensitive volume 36 in FIG. 6B experiences a magnetic field of approximately constant magnitude, but that rotates synchronously with the drill collar. It may appear that for hydrogen nuclei with longitudinal relaxation times T1 of several seconds, these spins would not align themselves properly with a rotating field that completes a rotation in much less time than it takes for polarization. However, the relaxation time T1 only governs the build-up of the magnitude of the nuclear polarization. A change in direction can be followed much faster and depends on the resonant Larmor frequency of the nucleus. At a typical field strength of 117 gauss, the resonance frequency is 500 kHz. The condition for Adiabatic Fast Passage (AFP), under which the nuclear polarization follows a change in direction of the polarizing field virtually instantaneously, is that the rate of change must be much less than the period of the Larmor frequency. Comparing 500 kHz with a maximum rotational frequency of 5/sec, we find a ratio of 100,000:1, which satisfies the requirement for an AFP condition by a wide margin. Consequently, although using a rotating drill collar which causes all fields within the formation to constantly change direction, the hydrogen nuclei always follow the changing directions without noticeable delay.
As is known in the art of NMR measurement, the frequency of the RF pulse must be tuned to the Larmor frequency (fL) (in Hertz) of the hydrogen protons, which is given by the following equation:
fL=4258 B0xe2x80x83xe2x80x83Eq. [6]
where B0 is the magnitude of the static magnetic field (in Gauss). Because the static magnetic field of this invention decreases monotonically as a function of radial distance from the tool, the location of the sensitive zone may be selected by choosing a value of B0 that coincides with the desired radial distance from the tool. In this manner, the sensitive zone can be fixed entirely within the earth formation to be measured, instead of partially in the borehole. As described in U.S. Pat. No. 4,350,955, issued Sep. 21, 1982, to Jackson and Cooper, if the sensitive zone is partially in the borehole, that situation presents a serious drawback to the system in that the NMR signal from the borehole fluid overwhelms the signal from the earth formation.
Prior art systems have attempted to solve this problem by doping the borehole fluid with chemicals (as described in the ""955 patent), which was time consuming and expensive, or by utilizing a gradient coil to produce an additional pulsed magnetic field to cancel the borehole signal (as described in the ""201 patent), which further complicated the system. Therefore, the capability of the present invention to fix the sensitive zone completely within the earth formation without any additional apparatus constitutes a valuable improvement over most of the prior art. Moreover, the static magnetic field of this invention as a function of radial distance from the tool is such that the location of the Larmor frequency for the sodium (Na) quadrupole moment lies inside the tool volume, as shown in FIG. 23. The gyromagnetic ratio of sodium is 1127 Hertz/gauss. To resonate at the same Larmor frequency, sodium requires approximately four times the field strength, a condition that is met at a diameter of about one-half of the sensitive diameter of hydrogen. The hydrogen diameter of 13.5 inches has been chosen to contain the potential sensitive volume for sodium with a diameter of 6.75 inches entirely within the tool. Therefore, this invention has the added advantage that it is not sensitive to sodium in the borehole fluid. It should be obvious to someone skilled in the art to scale the resonance diameters appropriately for tools used in different-sized boreholes.
The above-described advantage with respect to the completely-in-formation sensitive zone is made possible by combining the constant gradient tubular magnet with a nonmagnetic metal drill collar, an axially elongated antenna, high electrical conductivity shielding for the antenna, and a ferrite buffer. Although other practitioners in the art were of the opinion that this invention would not work with a metal drill collar, the inventors have demonstrated the contrary. A metal drill collar is desirable to increase the tool""s strength and durability in the harsh downhole environment of high temperatures, pressures, and abrasive fluids and particles and corrosive fluids. The antenna of this invention, which is used both to transmit the RF pulses and to receive the NMR signals, is located on the outside of the drill collar and, along with the magnet, has a relatively long axial dimension to produce a sensitive volume having a large axial dimension, as described above. In one embodiment, a shield made of high conductivity material (such as copper) is placed between the outer surface of the metal drill collar and the antenna to reduce acoustic ringing of the metal drill collar due to the RF pulses and to increase the efficiency of the antenna during the transmission of the RF pulses. The shield is thin enough to attenuate surface acoustic waves in the shield, yet it is fixed to the collar firmly enough to prevent bulk vibration of the shield. Further, the shield is acoustically isolated from the collar to prevent surface acoustic waves in the shield. These acoustic and vibration characteristics are enabled by bonding the shield to the collar with a thin layer of material having the desired acoustical properties, such as rubber or lead-filled epoxy. In another embodiment, the outer surface of the drill is made highly conductive, so that there is no need for a separate shield.
Further, in accordance with this invention, a layer of ferrite material is placed between the antenna and the shielding to direct the pulsed RF magnetic fields into the formation and to further increase the efficiency of the antenna so that it requires less power in the transmission mode and has increased sensitivity in the receive mode. Preferably, the ferrite material is layered such that a more or less continuous path through the ferrite material exists along the magnetic field lines of the RF field, but repeated discontinuities are introduced in the transversal direction to the RF magnetic field lines. These features in combination enable the placement of the sensitive zone far enough away from the tool to be completely in the formation yet still allow accurate sensing of the NMR signal by the antenna.
Yet another aspect of this invention involves a high-current, low-impedance feed-through connector to connect the antenna to the antenna""s tuning capacitors. As in the prior art, tuning capacitors are utilized in the antenna electronics (driving circuitry) to match the impedance of the antenna so that the antenna will resonate at the desired frequency. However, the capacitors are sensitive items and require protection from the high pressures and temperatures of the borehole environment. Before the invention described in U.S. Pat. No. 5,557,201, this problem was solved by selecting capacitors with minimal pressure and temperature sensitivities and isolating the capacitors from the borehole fluids in an oil-filled compartment of the drill collar. The compartment seal separated the compartment from the borehole fluids, but the seal did not form a pressure seal and therefore the compartment saw the ambient borehole pressure. Consequently, the compartment was filled with oil to transmit the ambient pressure uniformly around the capacitors and thereby prevent them from being crushed by the high differential pressure. Moreover, because the oil expands and contracts with changing temperature and pressure, these prior art devices had to include a means of varying the volume of the compartment to compensate for the temperature and pressure changes. Thus, such a scheme was very cumbersome.
The ""201 invention solved this problem by housing the antenna driving circuitry in a compartment that is not only sealed off from the borehole fluids but is also sealed off at constant atmospheric pressure. Thus, the compartment is simply filled with air instead of oil, and there is no need for a volume-regulation device. This method of protecting the capacitors makes the manufacturing of the tool much simpler and less costly. However, because the pressure in the vicinity of the antenna is much higher than the pressure in the capacitor compartment, the apparatus for feeding the antenna into the capacitor compartment must withstand a severe pressure differential. With such a high pressure differential, one would desire to minimize the area of the feed-through apparatus to minimize the force acting on it. On the other hand, because this NMR measurement-while-drilling (MWD) tool requires a very high current in the antenna, the area of the feed-through apparatus must be large enough to accommodate a conductor of sufficient size to meet the high current requirement. Additionally, the feed-through area must be large enough to supply a sufficient gap between the two antenna wires as well as to any surrounding metallic material.
This invention solves the problem posed by these conflicting area requirements by providing a conductor with a corrugated-shape cross-section for the feed-through connector. The corrugated shape of the conductor provides sufficient size to carry the high current, yet the conductor requires much less feed-through area for the connector than that which would be required for a conventional, flat cross-section conductor. Thus, this corrugated design minimizes the force on the feed-through connector while still accommodating the necessary current. Moreover, the corrugated design improves the bond between the conductor and the surrounding connector material by providing more bonding area. The connector maintains a stripline interface, thereby minimizing stray magnetic fields and electromagnetic losses.
Yet another aspect of this invention involves a method of mounting the electronics in the outer portion of the drill collar in such a way as to minimize the drill collar stresses that are transferred to the electronics. In the borehole, the drill collar is frequently subjected to bending stresses, axial stresses, and torsional stresses. Thus, while one side is in compression, the other side is in tension, and the highest stresses are in the outer portion of the drill collar. Because the drill collar rotates during drilling, the drill collar undergoes many cycles of tensile and compressive stresses. Therefore, any structure that is fixedly mounted to the drill collar will be subjected to similar strains as the collar material undergoes at the mounting surface, and the strains will be highest in the outer portion of the drill collar. Hence, the outer portion of the collar would seem to be an undesirable location to mount the delicate electronics.
This invention allows the installation of the electronics in the outer portion of the drill collar by means of a mounting scheme wherein the stresses of the drill collar are not appreciably transferred to the electronics. Specifically, one of the two ends of the electronics module is fixedly mounted to the drill collar, but the second end is mounted to the collar with a sliding connection. Thus, as the drill collar bends, elongates, and otherwise develops stresses, the second end of the electronics module slides relative to the drill collar and therefore the electronics module remains relatively free of the drill collar stresses. By virtually eliminating the transferred stresses, this invention allows the installation of the electronics in the outer portion of the drill collar and greatly reduces the potential strain and fatigue problems, such as broken printed circuit board traces and broken leads on components due to over-stress.
Finally, the NMR MWD tool embodying this invention requires very high downhole power within a very short time period. Specifically, it requires about 1.5 kilowatts during the short duration of an NMR measurement, but only a few watts between measurements For example, a typical load change would be 1 kW for 10 msec, 50 W for three seconds, 1 kW for 10 msec, and so on. Before this invention, existing downhole power generators were not directed to meeting these kind of fluctuating power requirements. Therefore, the apparatus embodying this invention includes a high power generator to meet the need.
Accordingly, it is an object of this invention to provide an improved apparatus and method for performing a fall range of NMR measurements on formations surrounding the borehole in particular while drilling that address the above issues and overcome deficiencies associated with the prior art.